It has been known for some time that transition metal alloy systems and especially titanium-containing alloys, particularly Laves phase alloys AB.sub.2, in which A represents a component which includes titanium and possibly another element from the early part of the transition elements and B represents an element from the later part of the transition elements of the Periodic Table, can be utilized effectively as a hydrogen storage medium. Such compositions are utilized for the safe storage of hydrogen, e.g. for use as fuels, the composition being charged with hydrogen and therafter serving as a source for the controlled release of hydrogen in a manner which is safer than that which obtains when the hydrogen is supplied from high pressure cylinders or the like.
In such compositions, the elements are so chosen that the atomic radius ratio r.sub.A /r.sub.B is between 1.05 and 1.68, r.sub.A representing the atomic radius of atoms of the elements constituting the A component, namely, titanium and at least one other element from the beginning of the transition metal series of the Periodic Table while r.sub.B represents the atomic radius of the elements of the B component, i.e. one or more elements from the remainder of the transition metal series of the Periodic Table.
In the past such alloys have been made by preparing the elements in sufficient purity and then smelting them in a vacuum furnace and/or under protective gas to form a prealloy.
The prealloy is then comminuted and subjected to smelting again in the vacuum furnace in a second stage of the process.
The hydrogen storage alloys which result thus have intermetallic phases which are equivalent to chemical compounds formed between the components with stoichiometric proportions of the components characterized by the overall relation AB.sub.2. A and B can each be a single element or can represent a group of elements and such stoichiometry exists between the sum of the elements of one group and the sum of the elements of the second group when the ratio of the atomic radii alloy within the range given.
These compounds or intermetallic phases crystallize in a so-called C-14 structure, which has an especially dense atomic packing and together with C-15 and C-36 structures in the crystal composition has been designated as a Laves phase structure.
The elementary cell of the C-14 structure is hexagonal and contains four A-component atoms and eight B-component atoms. The compounds have a metallic appearance and typical compositional characteristics and properties of such hydrogen storage alloys which are given below.
It should be clear that an entire collection of hydrogen storage alloys of this type has been developed, these alloys including apart from titanium and manganese, which are the components present in practically all of them, vanadium, chromium and iron and/or aluminum.
These alloys are characterized by a high hydrogen storage capacity which can exceed 2% by weight H.sub.2. The hydrogen storage capacity is defined as the difference in weight percent H.sub.2 between the hydrogen uptake at room temperature and 50 bar H.sub.2 pressure and the residual H.sub.2 content at 60.degree. C. and 1 bar H.sub.2 pressure.
It is important for the use of such alloys as sources of hydrogen, that the pressure plateau over the concentration range be as horizontal as possible and that this horizontal stretch be maintained over a wide concentration interval.
In the past, hydrogen storage capacities in the high level of say to 8% H.sub.2 have generally been achieved only under extreme conditions in laboratory tests and generally are not realizable in large scale practice.
In German open application DE-OS 30 23 770, which describes the prior art method discussed above, all of the constituent elements including titanium are already present in the first stage melt. The first stage product is a prealloy only in the physical sense with respect to structure and possibly disadvantageous homogeneity characteristics since it already has the chemical composition of the final alloy so that the term "prealloy" may be a misnomer.
After comminution, the composition is smelted again and the second smelting stage serves mainly for homogenization having little effect on the alloy composition although it has an important effect in providing the hydrogen storage capacity. It appears that this two-stage process ensures an especially low oxygen content and this contributes to the improvement in the hydrogen storage capacity since oxidic impurities appear to be detrimental to the hydrogen storage capacity.
Naturally, this also requires that the starting materials be extremely pure.
For the production of hydrogen-rich alloys, it is desirable to make use of commercially available metals and alloys, especially titanium sponge, zirconium sponge, electrolytic manganese, ferrovanadium, vanadium metal, electric furnace or electrolytic iron, cerium mischmetal and vanadium-aluminum alloys. All of these elements can be produced by conventional smelting metallurgy or powder metallurgy, but must be treated in vacuum and/or under protective gas for smelting or sintering.
The smelting units include induction furnaces, electric-arc furnaces and electron-beam furnaces and, to avoid the incursion of impurities, it is preferred to operate in a crucible-free mode thereby avoiding reactions with the crucible materials.
Impurities in the form of oxide products, such as Al.sub.2 O.sub.3, but even carbon, have been found to reduce the quality and adversely affect the hydrogen storage capacity.
Best results are thus obtained when one makes use of an electric arc furnace and carries out the smelting in a protective gas with the second stage smelting being effected in a water-cooled copper ingot mold.
In spite of all of these efforts, however, one generally obtains a hydrogen storage composition as the product which has a relatively high oxygen concentration. As a result, the hydrogen storage capacity is significantly lower than the aforementioned goal of two weight percent hydrogen.
If one does not take the precautions described above to ensure deoxidation of the alloy, the oxygen content can range from 0.4 to 0.6 weight percent with a corresponding reduction in the hydrogen storage capacity.
Our tests have shown that deoxidation to reduce the oxygen/oxide content of the alloy is highly problematical because the removal of the partially solid and partially viscous deoxidation products from the melt is extremely difficult.